In radio communication systems, at least a portion of the communication between two communication terminals which are connected by means of the radio communication system takes place via an air interface, that is, via a radio connection. Such an air interface has only limited radio resources. The radio resources, such as the extent of the frequency spectrum available and the time available must be assigned particularly effectively, in particular for the planning of a radio network.
The GSM mobile radio network is such a radio communication system in which the air interface lies between fixed base stations and mobile stations and regulates the communication between them. In this case, a base station has a plurality of transmission devices which each supply one radio coverage area with radio resources and which are provided for the communication with the mobile stations located in this radio coverage area. The radio resources of a transmission device constitute the frequency channels, at least one of which frequency channels is required for transmitting the system information and the time slots within a frequency channel which may be distributed over a plurality of mobile stations, that is to say over a plurality of communication terminals.
In a GSM network, area coverage is usually aimed at so that the radio coverage areas of the transmission devices of a base station as a rule form a zone of continuous coverage around the base station. The frequency channels used by the transmission devices in a base station must differ from one another in terms of frequency in order to prevent interference, in the same way as the frequency channels must differ from the frequency channels of adjacent radio coverage areas of other base stations, that is to say of other radio cells. Since only a specific bandwidth in the frequency spectrum is available to a network operator for the radio communication system, the operator is faced with the complex task of effectively apportioning the available radio resources, usually the frequency channels, to the base stations, and ultimately transmission devices, taking into account the traffic load values.
If the radio resources of a transmission device are completely utilized and there is a new resource request, for example, for a further call setup, additional spectrum resources must be made available to this radio coverage area. However, with the time slot multiplexing method, an additional frequency channel for this frequency coverage area means that a considerable overcapacity of radio resources (here time slots), which is not used, is offered for one call-setup request. This offered overcapacity for only one radio coverage area leads, in particular in the case of base stations in regions with low usage of the radio network, for example in rural regions where only a few frequency channels have to be offered, to a significant reduction in the efficiency of the utilization of the offered radio resources and thus to an ineffective apportionment of the radio resources.
Motorola "BSS Equipment Planning Guide" dated 1992 discloses a solution which makes it possible for the modules used to be utilized more efficiently. This is achieved in that an additional transmission device is assigned to a base station, which transmission device is capable of transmitting into, and receiving from, all the radio coverage areas realized by the base station and its transmission devices. However, a new additional frequency, that is to say an additional frequency channel, is assigned for each radio coverage area of the base station. The additional transmission device must therefore switch over between the frequency channels in accordance with the time slots. Thus, although the additional transmission device is utilized more efficiently since it is possible to fulfil resource requests from all the radio coverage areas associated with the base station, extensive utilization of additional spectrum resources (three additional frequency channels) occurs. The spectrum resources thus continue to be utilized inadequately.